Photoelectric cells



Aug. 22, 1961 M. LUBlN PHOTOELECTRIC CELLS Filed March 15, 1957 INYENTORMam/1a Mbm U M ATTORNEYS United States Patent 2,997,677 PHOTOELECTRICCELLS Marvin Lubin, Oak Park, Ill., assignor to Hupp Corporation,Cleveland, Ohio, a corporation of Virginia Filed Mar. 15, 1957, Ser. No.646,318 14 Claims. (Cl. 338-15) This invention relates to photoelectriccells and more particularly to an improved cell housing and method ofmanufacture thereof for photoresistance semiconductor elements, and is acontinuation-in-part of my co-pending application Serial No. 538,573filed October 5, 1955, now abandoned.

It has been found that the mounting structure for cadmium sulfidecrystals plays such an important role in the operation of the cell thatotherwise satisfactory crystals are rendered Worthless by impropermounting in housings. For example, the crystal ages very rapidly inimproper types of housings to lose its desirable operatingcharacteristics in a matter of a few weeks even though initial testsshowed the crystal to be fully satisfactory. Since one of the principaladvantages of the cadmium sulfide crystal resides in its high currentcarrying capacity which is large enough to operate electromagnet coilsdirectly without further current amplification, the importance of thecell in maintaining its high current rating is one of primarysignificance.

It has been observed in the laboratory that cadmium sulfide crystals arecapable of passing much larger currents than can safely be passed 'bythe completed photocell with a structure enclosing such crystals.Housings to enclose the crystal are, however, essential to preventcontaminating effects caused by various elements in the atmosphere.Since the cells are commonly used in circuits where a constant voltageis applied, illumination of the cell causes a reduction in resistanceand corresponding increase in current which, in general, increases thequantity of heat that must be dissipated by the crystal to a maximumvalue when the current flow is maximum. Increased temperature of thecell causes an increase in the resistance and hence a lower operatingcurrent which renders the cell unsatisfactory for many applications. Asecond harmful effect of crystal heating is the development ofmechanical stresses and strains due to different rates of expansion ofthe various components in the photocell. Thus the connection of theelectrical terminals to the crystal are weakened at their point ofcontact thereby reducing maximum sensitivity and causing furtherlocalized heating which shortens the life especially at high currentlevels of operation.

It is therefore a major object of the present invention to provide animproved housing for a cadmium sulfide or like crystal which has meansfor adequately and effectively dissipating the heat generated within thecrystal without compromising any of the other factors neces-.

sary for satisfactory operation and long life.

Another object of this invention is to provide a heat dissipationsurface of substantial extent which is supported at a position remotefrom the base of the photocell to improve the efficiency of heatremoval.

A further troublesome problem involves the electrical nature of thecontact to the crystal. It has been found this problem has beensubstantially eliminated by first cleansing a spot on the crystal byglow discharge as dis closed in my above-identified copendingapplication and then gold plating the cleansed spot. By using a silvercement or similar adhesive between the gold spot and a small Wireforming one terminal on the crystal, there resulted an enormousimprovement in crystal characteristics including reduction of contactnoise, reduction of the rectifying effect and improved linearity betweenvoltage and current. There still remained however,

Patented Aug. 22, 1961 ice pact housing with rigid electrical terminalsand which lends itself better to mass assembly techniques.

. It is accordingly a further object of this invention to provide animproved crystal housing which retains all of the advantages ofoperation provided by my prior crystal housings and which is readilyadapted to mass production techniques.

A further object of the present invention is to provide a novel housingwherein the terminal leads are supported in a body of moldableinsulating material and the radiation sensitive body of crystallinematerial is supported on a plate of thermally and electricallyconducting material having a large surface area exposed to theatmosphere.

Another object of the invention is to provide an improved photocellwherein the electrical terminals are molded between opposite faces of aplate-like body of insulating material which is formed with an apertureand the body of radiation sensitive crystalline material mounted in saidaperture on one surface of a plate of thermally and electricallyconducting material and the latter plate secured to the plate ofinsulating material to provide a large radiating surface for carryingaway heat dissipated by the crystalline material. As a further feature,the aperture is filled with a neutral plastic material to protect thecrystal from contamination by atmospheric elements.

A still further object of the invention resides in the novel method ofassembling the photocell housing of the present invention which includesfirst embedding the elecinert plastic material transparent to theradiation to be q tive-to the edges of the metal mounting plate wherebythe electrode connection to the spot may be made automatically bymachine operated equipment during mass assembly of the crystal housings.

These and other objects of the invention will become more fully apparentby reference to the appended claims and the following detaileddescription when read in conjunction with the accompanying drawingswherein:

FIGURE 1 is an enlarged perspective view of a photocrystal as used inthe present invention;

FIGURE 2 illustrates diagrammatically a form of apparatus suitable forcleaning crystal surfaces and forming the gold layer electrodes thereon;

FIGURE 3 is a perspective view showing the crystal mounted on the plateof thermally and electrically conductive material as it appears at onestage in the production of the photocell in accordance with the presentinvention;

FIGURE 4 is a side view showing the sub-assembly comprising the twoelectrodes embedded in a body of insulating material as it appearsbefore the metal mounting plate as shown in FIGURE 3 is secured thereto;

FlGURE 5 is a front view of a completed photocell in accordance with thepresent invention;

FIGURE 6 is a side view in section of the completed, photocell of FIGURE5; and

FIGURE 7 is a rear view of the photocell shown in FIGURES 5 and 6.

With continued references to the drawings wherein like referencenumerals are used throughout to designate like elements, thephoto-crystal designated generally by reference numeral and asillustrated in FIGURE 1 is in the form of a small single crystal or bodyof photosensitive semiconducting material which preferably is cadmiumsulfide. Crystal 10 may be grown in a form of a single crystal by thebasic methods as disclosed in copending application Serial No. 486,927filed February 8, 1955 by Leonard E. Ravich, now Patent No. 2,890,939,and assigned to the same assignee as the present invention, thoughcrystals produced by other methods may be also used. Cadmium sulfidecrystals of this type generally range in size from a few squaremillimeters to several square centimeters in area and usually range froma few tenths of a millimeter to several millimeters in thickness.Satisfactory results are obtained in mounts of the present inventionwith crystals as small as 0.3 centimeter on each side and with the usualthickness of a fraction of a millimeter. Photocells constructed inaccordance with the present invention have at least as great a currentcarrying capacity under comparable voltage and illumination conditionsas cadmium sulfide crystals offered in the commercial market by othersof which I am aware which have 40 times the size of sensitive area asthe above exemplary dimensions.

For example, photocells of the present invention have a dark resistanceof 50 to 500 megohms. With 45 volts D.C. applied across the terminalsand an illumination of 50 foot candles, a current from 30 to 100milliamperes is obtained thereby providing a resistance of from about450 to 1500 ohms and a light to dark ratio of from 10 or 10 To obtainthe characteristics described above, photo element 10 as shown in FIGURE1 is first treated by ion bombardment and the bombarded area plated withgold. A simple form of apparatus for so treating crystal 10 isillustrated in FIGURE 2 to which reference will now be made.

In FIGURE 2, a bell jar 22 and a base plate 24 on which it sealstogether define the vacuum chamber from which the atmosphere may beevacuated through an exhaust line 26 to roughing and holding vacuumpumps (not shown). If desired, an inlet line 28 also may be provided forflushing the chamber with a selected gas or vapor prior to the cleaningand plating operations described below.

Within the vacuum chamber, the photocrystal blank 30' is mounted in amask and a holder assembly 32 which is generally of the type shown inFIGURE 3 in my aboveidentified copending application which is preferablyrotatable by suitable means 34 fixed to the outer end of a mounting stem36 through a seal 38 in the bell jar wall. Mounting stem 36 is made ofmetal and grounded as indicated by reference numeral 49. Electrode 40 ismounted above mask 32 and is provided with an electrical lead 42 to theexterior of the bell jar. Below the mask and holder assembly, a strip orfilament type heating element 44 is mounted between an electricallyconnected pair of lead wires 46 and 47 which extend to the exterior ofthe bell jar for connection across a battery or other electrical currentsource which is capable of passing sufficient current through heaterelement 44 to raise it to a temperature adequate to vaporize goldmaterial 48 placed in or on the heater element as illustrated.

The cleaning and electrode plating operations are car- 4 the preferredmethod of assembly will be secured to the thermally conducting plate 50.Then the roughing pump is connected to evacuate the bell jar. Pressurewithin the bell jar preferably is first reduced to between 0.05 and 0.1millimeter and upon reaching this pressure a glow discharge betweencrystal 10 and electrode 40 is initiated by energizing coil 43 with ahigh alternating voltage in the range of about 10,000 to 14,000 volts.The residual gas ions are accelerated toward and impinge upon theexposed upper surface of crystal 10 where they have the effect ofreducing or eliminating the surface insulating barrier which ischaracteristic of the untreated crystal and which probably is due to thepresence of adsorbed gases. The length of time during which ionbombardment is continued depends on the particular application for whichthe crystal is intended; in general a glow discharge continued forapproximately 10 minutes is effective to completely orat leasteffectively remove the surface barrier.

After ion bombardment of one side of the crystal is completed, the glowdischarge voltage is disconnected andthe roughing pump closed off and,without breaking the vacuum in the bell jar, the holding pump isconnected to further reduce the bell jar pressure preferably to between0.05 and 0.1 micron. The crystal 10 is then turned as by rotation ofholder stem 36 so as to place the cleansed surface of crystal 10 in aposition above and facing heater 44.

With the pressure held between these approximate limits, heater elementleads 46 and 47 are connected to a current source to raise thetemperature of the heater element 44 sufficiently to vaporize the goldmaterial 43 previously placed thereon onto the downwardly facing surfaceof crystal 10. This electrode deposition operation is continued untilthe electrode is built up to a desired thickness. In the preferredembodiment the size of this gold electrode is a spot having a diameterof at least 0.060 inch. Since the same mask is used, the metal electrodeis coextensive with the area cleansed by the ion bombardment treatment.g

In mass production assembly, the crystal is then re} moved from hell jar22and placed on a mounting plate 50 as illustrated in FIGURE 3. Mountingplate 50 is made of thermally and electrically conducting material suchas copper, and a thin coating of silver on plate 50 is preferred toprevent oxidation of the copper and because of the improved electricalnature of the contact between the gold plated electrode on crystal 10and the silver coating on plate 50.

Crystal 10 is then placed centrally between the side edges 52 and 54 ofplate 50 and near end 56 for purposes which will become apparent as thedescription proceeds. A jig or other suitable equipment (not shown) ispreferably used to provide uniform positioning in mass assem- 1 bardmentcleaned as described above.

ried out sequentially, preferably in the steps of selecting blyproduction. The crystal is then soldered or otherwise secured with anadhesive having a high metallic content to the silver plated copperplate 50 with the gold plated surface facing plate 50.

Plate 50 with crystal 10 attached to it is then placed in a bell jarequipment such as that shown in FIGURE 2 and a suitable mask providedexposing a small area of approximately 0.030 inch in diameter on crystal1 0. A vacuum is produced and the exposed surface bom- Then the holderis turned over without breaking the vacuum and a gold plating applied tothe cleansed spot. The mask contains suitable positioning surfacescooperating with edges 52, 54 and 56 to accurately locate the maskaperture uniformly on each unit. Plate 50 with crystal 10 attached to itis then removed from the bell jar and is ready for assembly with theremainder of the photocell housing next described below.

Referring now to FIGURE 4, the principal element of the mount is a body62 of moldable insulating ma-' terial such for example as Bakelitewhich, in the illustrated embodiment has agenerally rectangular shapewith acetate a thickness much smaller than either of the dimensions ofthe rectangular front and rear faces. Before body 62 is formed, a pairof rigid wires 64 and 66, also shown in FIGURES though 7, are placed inthe mold. Wire 64 is provided with a right angle bend to cause end 68 toextend outwardly beyond the material in the mold. Wire 66 extendsstraight through body 62 and both wires 64 and 66 lie in a central planethereof. Body 62 is formed with a recessed rear wall 70 surrounded withridge surfaces 72, 74 and 76. A through aperture 80 is provided in body62 with a conical counter sink 82 adjacent rear wall 70. Aperture 80 issufficiently large so that section 81 of conductor 66 is exposed andbare as best shown in FIGURE 5. A front ridge 84 may be provided ifdesired thereby serving as a lip extending around the walls formingaperture 80.

After body 62 is molded with leads 64 and 66 in place so as to providerigid terminals whereby the crystal housing may be placed in a suitablesocket, metal mounting plate 50 with crystal is placed in the recessagainst rear wall surface 70 with crystal 10 in aperture 80. Anysuitable adhesive may be used to permanently secure metal plate 50 torear surface 70. Projection 68 shown in FIGURE 4 on conductor 64 is cutoff flush with rear surface 90 of metal plate 50 and electricallyconnected to metal plate 50 as by a solder connection 91 (see FIG- URE6).

Since in mass production the crystal is accurately located on metalplate 50 by means of a suitable jig, crystal 10 is uniformly locatednear the center of aperture 80 with the exposed gold plate spot 59accurately positioned on each crystal at precisely the same location ineach mounting structure, which for example may be at the center ofaperture 80. This facilitates making the electrical connection betweengold plated spot 59 and section 81 of lead 66. This connection ispreferably in the form of a small flexible piece of Phosphor bronze wire92. Wire 92 preferably is first soldered to gold plated spot 59 oncrystal 1t) and then the wire bent over and soldered to section 81 oflead 66. Aperture 80 is subsequently filled with a suitablethermoplastic material 94 transparent to radiation to be detected andchemically inert to the crystal material, which for example may be athermal setting polyester such as Kelon or Selectron. The thermoplasticmaterial 94 runs underneath conical surface 82 thereby assuringpermanency of the assembly. The front surface 96 of thermoplasticmaterial protrudes as a slight convexity and may be either left in thatshape to take advantage of such focusing action as may be provided orpolished smooth to be flush with ridge 84 as illustrated in FIGURE 6.

The advantages resulting from the crystal housing of the type describedabove are several. By having plate 50 on an outer surface of theprincipal housing body 62, a large surface area is provided forefficient removal of heat generated within crystal 10. It may bepreferred in some installations to provide ridges 98 illustrated inFIGURE 7 on the surface plate 50 exposed to the ambient atmosphere toincrease the efiiciency of the heat removal though this is not necessaryin all units. The large area gold electrode and solder (or cement havinga high density of metal) securing crystal 10 to plate 50 serves as acontinuous metallic connection between the crystal and the relativelylarge, highly thermally conducting heat radiating surface and causes theheat developed in the crystal to be efliciently distributed throughoutplate 50 thus preventing localized regions of high temperatures. Testshave shown that for two cells both carrying the same current, that onecompletely encapsu lated in a thermoplastic material while the other isof the form herein described, a temperature difference on the order of40 C. at the crystal is obtained. Thus the mount of the presentinvention is extremely effective in maintaining a low crystaltemperature.

In addition, with the use of a solder contact between the goldevaporated electrodes on the crystal and the adjoining circuit on bothsides of the crystal connections, the improved electricalcharacteristics of the contact are obtained as the rectification effectand noise are both greatly reduced and the voltage-current linearitybecomes substantially perfect. Preliminary field tests on the housing ofthe present invention lasting over a period of four weeks have shown nofailures under conditions where approximately 50% of the older typecells, which were completely encapsulated in a plastic, failed.

The improved crystal housing of the present invention is especiallyadapted to be inserted in circuits by means of sockets similar to vacuumsockets. Leads 64 and 66 are rigidly secured in the principal housing 62which is easily grasped for a desired manipulation of the crystalhousing and provides an extremely rugged construction whereby nophysical damage occurs to the housing or change in operatingcharacteristics is caused by the usual handling given electroniccomponents. Moreover, the housing supports the sensitive materialapproximately of an inch from the socket into which leads 62 and 64 aremounted in a position to receive radiation energy in a planeperpendicular to the direction of leads 62 and 64. Metal mounting plate50 is also advantageously supported away from the socket receiving leads62 and 64 to permit air to circulate freely across its exposed surfacefrom which heat generated within crystal 10 is removed.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States LettersPatent is:

1. In a housing formed of a body of insulating material for aphotosensitive element of crystalline cadmium sulfide, a supporting baseof good electrical and heat con-' ducting material large relative to thesize of said photosensitive element positioned on an exterior surface ofsaid housing, means mounting said photosensitive element to have oneface in a good thermally and electrically conducting relation on onesurface of said base, a first conductor electrically connected to saidbase at a point remote from said photosensitive element and providing atits opposite end a rigid terminal, a second conductor electricallyinsulated from said base by said body of insulating material and havinga rigid end parallel to and spaced from said other end of said firstconductor forming socket terminals on said housing, and a small'flexiblewire mechanically secured and electrically connected to said secondconductor and having its free end electrically connected to a secondface on said photosensitive element.

2. In a mount for a photosensitive body of crystalline cadmium sulfidecomprising a housing including a body of insulating material, a base ofgood electrical and heat conducting material large relative to the sizeof said photosensitive body positioned on an exterior surface of saidhousing, means mounting said photosensitive body to have one face in agood thermally and electrically conducting relation on one surface ofsaid base, a first rigid conductor supported by said housingelectrically connected at one end to said base and providing a rigidterminal at its free end, a second rigid conductor electricallysupported by said housing to be insulated from said base and said firstconductor and having a free end parallel to and adjacent the free end ofsaid first conductor, and a flexible wire mechanically and electricallyconnected to said second conductor at a position adjacent said body ofcadmium sulfide and secured against a second face of said photosensitivebody.

3. A photocell comprising a housing of insulaitng ma terial having apair of electrical terminals at one end, a body of good electrical andheat conducting material supported on an exterior surface of saidhousing and having an exposed outer surface of substantial exfent at aposition remote from said one end, an aperture in said housingcommunicating with said body of conducting material, a slab ofcrystalline material sensitive to radiation sup? ported on said body ofconducting material and positioned in said aperture, and meanselectrically connecting said slab between said electrical terminals.

4. In a radiation sensitive device having a housing comprising a body ofinsulating material having a through aperture, a pair of electricalconductors imbedded in said insulating material with one of saidconductors extending into said aperture and with free end portions oneach of said pair of conductors extending outside said body ofinsulating material in parallel directions perpendicular to the axis ofsaid aperture, 2. base of good electrical and heat conducting materiallarger than said aperture secured to said body of insulating materialwith one face thereof exposed to the atmosphere and extending over saidaperture, a body of crystalline material sensitive to radiation mountedwith one face in good heat and electrical conducting relationship to asecond surface of said base of conducting material and positioned withinthe aperture in said body of insulating material, a fierdble wireelectrically connected between said one of said conductors and a secondface on said body of crystalline material, and means connecting theother of said pair of condoctors to said base.

5. In a device of the type described having a housing comprising a bodyof insulating material having a through aperture, a pair of electricalconductors embedded in said isolating material with one of saidconductors extending into said aperture, a plate of good electrical andheat conducting material with faces on opposite sides thereof havingdimensions in two directions substantially larger than the thickness ofthe plate between said opposite faces secured to said body of insulatingmaterial having one of said faces on the outside of said body andcovering the aperture therein, a slab of crystalline material sensitiveto radiation mounted to have one surface in good heat and electricalconducting relation on the other of said faces of said plate of heatconducting material and positioned in said aperture in the body ofinsulating material, an electrical conductor electrically connected tosaid one of said pair of electrical conductors and contacting an exposedsurface on said slab of crystalline material, and the other of said pairof conductors being electrically con nected to said plate of conductingmaterial.

6. The device as defined in claim further comprising sealing meanstransparent to the radiation to be detected covering said aperture onthe side of said body of insulating material opposite from said plate ofconducting material for protecting said slab of crystalline materialfrom atmospheric elements.

7. A radiation sensitive device comprising a flat body of insulatingmaterial having faces on opposite sides with the smallest dimension ofsaid faces being considerably larger than the thickness of the bodybetween said faces, an aperture extending between the faces in saidbody, a pair of electrical conductors imbedded in said body between saidfaces with one of said conductors extending into said aperture and theotherof said conductors extending through one of said faces, aplate-like body of good heat and electrical conducting material withsurfaces on opposite sides thereof and assembled with one of saidsurfaces secured to said one of said faces on said flat body so as tocover said aperture and be in electrical contact with said other of saidconductors, a slab of crystalline material sensitive to radiationmounted with one side in good electrical and heat transferring relationto said one surface of said plate-like body of conducting '8 materialand positioned in said aperture, a lead connected between said one ofsaid conductors and the exposed sur face of said crystalline slab, andmeans connecting said other of said conductors to said body ofconducting ma terial.

8. The device as defined in claim 7 further comprising sealing meanstransparent to radiation to be detected in the aperture of saidinsulating plate protecting and preventing contamination of saidcrystalline slab from atmospheric elements.

9. In a photosensitive device, a body of cadmium sulfide material havinga substantially flat surface of appreciable area on one side, anelectrode comprising .a layer of metal formed as a plating on saidsurface to substantially cover said surface, a massive slug of highthermal conductivity material considerably larger than said body ofcadmium. sulfide material, means mechanically securing said body ofcadmium sulfide material to said massive slug with said electrode layerforming a high thermal and electrical conductivity connection betweensaid body and said slug for maintaining the temperature of the body ofcadmium sulfide when carrying current as low as possible.

10. A photocell comprising a housing and a massive slug of highlythermally conducting material secured to said housing in a manner toradiate heat from said slug into the atmosphere surrounding saidphotocell, a body of cadmium sulfide having a substantially flat surfaceon one side, a layer of gold plating on said fiat surface, meansincluding a substance having a high metal concen tration for securingsaid gold layer to one side of said massive slug to provide a continuousmetallic connection between the body of cadmium sulfide and the heatradiating surface of the photocell exposed to the atmosphere, at firstelectrical terminal connected electrically to said slug and extendingoutwardly from said housing, a second electrical terminal extendingthrough said housing and insulated from said first electrical terminal,and means connecting said second electrical terminal to a second side ofsaid body of cadmium sulfide opposite said substantially flat surface.

11. The photocell as defined in claim 10 wherein said last mentionedmeans comprises a layer of gold plating applied to said second side ofsaid body of cadmium sulfide and said second electrical terminal issecured in good electrical relationship to said gold plating.

12. The photocell as defined in claim 10 wherein the mrface on saidsecond side of the body of cadmium sulfide is cleansed and plated withgold over at least a portion of its area and the second electricalterminal is connected to said gold plating whereby noise andrectification are reduced and the voltage-current linearity is greatlyimproved.

13. A photocell comprising a housing having an opening, a massive slugof highly thermally conducting material secured to said housing in amanner whereby heat from said slug is radiated into the ambientatmosphere surrounding the photocell, a generally flat body of cadmiumsulfide having one side of substantial area plated with a metal andsecured in the opening of said housing to said massive slug therebyproviding a continuous metallic connection between said body of cadmiumsul fide and the heat radiating surface of the photocell, a pair ofelectrical terminals electrically connected to op posite sides of saidbody of cadmium sulfide and extending exteriorly of said housing, and abody of encapsulating plastic material transparent to the radiation tobe detected in said opening sealing said body of cadmium sul-,

fide from the atmosphere but covering only part of said massive slug.

14. A high current photocell comprising a casing member having a hollowinterior portion, a relatively massive slug of high thermal conductivitymaterial supported by said casing member, a body of photosensitivesemiconductor material containing a large area metallic electrode on atleast one surface thereof, said body being disposed in the hollowinterior portion of said casing member with its metallic electrode inelectrically and thermally conductive relation to said slug, and meansincluding cooling 5 fins exposed outside of said casing member and inthermally conductive relation to said slug.

References Cited in the file of this patent UNITED STATES PATENTS ClarkApr. 19, 1938 Anderson et al Apr. 6, 1954 Anderson et a1. June 21, 1955Jacobs et al Aug. 20, 1957

